1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier or a printer, having a duplexer that is capable of duplex printing, and more particularly to a cooling mechanism for cooling a non-paper feeding part of a fuser roller pair.
2. Description of Related Art
A schematic configuration of a conventional image forming apparatus is shown in FIG. 10. In FIG. 10, reference numeral 100 represents an image forming apparatus. Here, a digital multifunction apparatus will be described as an example of the image forming apparatus. Assume that a copy operation is performed with the image forming apparatus 100. Then, in an image forming portion 3 provided inside the multifunction apparatus body 2, a small diameter photoconductive drum 5 rotating in a direction indicated by arrow A shown in the figure is uniformly charged by a charging unit 4, then an electrostatic latent image is formed on a photoconductive drum 5 with a laser beam emitted from an exposure unit (a laser scanning unit, or the like) 7 based on original image data read by an image reader 6, and then developer (hereinafter, toner) is attached to the resultant electrostatic latent image by a developer unit 8. In this way, a toner image is formed. The toner is supplied to the developer unit 8 from a toner container 9.
Toward the photoconductive drum 5 on which the toner image is thus formed, a sheet of paper is transported from a paper feed mechanism 10 to the image forming portion 3 by way of a paper transportation path 11 and a resist roller pair 12. In the image forming portion 3, the toner image formed on the surface of the photoconductive drum 5 is transferred to the sheet of paper by a transfer roller 13 (transfer means). The sheet of paper to which the toner image is transferred is separated from the photoconductive drum 5, and is then transported to a fuser 14 having a fuser roller pair 14a, where the toner image is fused to the sheet of paper. The sheet of paper that has passed through the fuser 14 is sent to a multi-branched paper transportation path 15, where path switch mechanisms 21 and 22 provided at branching points thereof and each having a plurality of path switch guides determine in which direction the sheet of paper is transported. Then, the sheet of paper is directly (or, after having been sent to a duplex-printing paper transportation path 16 to perform a two-sided copy) ejected onto a paper catch portion of a first output tray 17a or a second output tray 17b through an output roller pair 20a or 20b. 
Though not shown in the figure, a discharger for removing charge remaining on the surface of the photoconductive drum 5 is disposed at a downstream side of a cleaner 18 along the photoconductive drum 5. Furthermore, the paper feed mechanism 10 is detachably attached to the multifunction apparatus body 2, and is composed of a plurality of paper feed cassettes 10a and 10b that hold sheets of paper, and a stack bypass (a manual feed tray) 10c. The plurality of paper feed cassettes 10a and 10b and the stack bypass 10c are connected by way of the paper transportation path 11 to the image forming portion 3 built with the photoconductive drum 5, the developer unit 8, and the like. Reference numeral 24 represents a platen (a document retainer) that retains a document placed on the image reader 6.
Specifically, at a downstream side of the fuser roller pair 14a, the paper transportation path 11 first forks into two paths, of which one (in FIG. 10, a rightward path) is connected to the first output tray 17a via the output roller pair 20a, and the other (in FIG. 10, a leftward path) further branches into two paths, of which one is made to eject a sheet of paper into the second output tray 17b via the output roller pair 20b and the other is connected to the duplex-printing paper transportation path 16. Incidentally, the duplex-printing paper transportation path 16 is a path along which a sheet of paper having an image on one side thereof is switched back and then transported. After image formation is repeated by the image forming portion 3 on the other side of the sheet of paper thus transported, the resultant sheet of paper is ejected (a two-sided copy is made).
In such an image forming apparatus, the fuser roller pair 14a needs to have a uniform temperature distribution so that the fuser 14 can stably fuse a toner image. However, when a sheet of paper passes through the fuser roller pair 14a, the fuser roller pair 14a is deprived of heat where it makes contact with the sheet of paper (a paper feeding area). This causes accumulation of heat in the area of the fuser roller pair 14a where the sheet of paper does not pass thorough (a non-paper feeding area) because the fuser roller pair 14a is not deprived of heat in a non-paper feeding area.
FIG. 11 shows how the fusing temperatures of a paper feeding area and a non-paper feeding area change when printing is continuously performed on sheets of paper whose width is smaller than the maximum width of the fuser roller pair. Here, as an example of implementation, a description will be given of a case where A5 sheets of paper are continuously fed to the apparatus. At the start of continuous printing, the temperature of a paper feeding area (indicated by a heavy line in the figure) is set around 200° C., and the temperature of a non-paper feeding area (indicated by a thin line in the figure) is around 185° C., which is slightly lower than that of the paper feeding area, due to the arrangement of heat sources inside the rollers, the influence of outside air, or the like.
When 50 seconds have elapsed after the start of continuous printing, the temperature of the paper feeding area drops to about 175° C. due to the contact of the sheets of paper. To prevent poor fusing resulting from this temperature drop, the output of the heat sources is increased, causing gradual increase in the temperature of the paper feeding area. However, since the non-paper feeding area is not deprived of heat by the paper, the temperature thereof rises more sharply than that of the paper feeding area. As a result, when 300 seconds have elapsed, the temperature of the non-paper feeding area is more than 20° C. higher than that of the paper feeding area.
When the continuous feeding of paper is temporarily stopped after a lapse of about 330 seconds, the temperature of the paper feeding area rises again to around 200° C. This reduces the difference between the temperatures of the paper feeding area and the non-paper feeding area. However, once the continuous feeding of paper is resumed, the temperature of the paper feeding area drops again for the reason mentioned above. Inconveniently, repeating this cycle causes unnecessary rise in the temperature of the non-paper feeding area and shortens the life of the fuser roller pair. On the other hand, if the temperature of the fuser roller pair is as a whole reduced with an increase in the temperature of the non-paper feeding area, the temperature of the paper feeding area is also reduced. This makes it impossible to supply to a sheet of paper the amount of heat required to fuse an image thereto.
To overcome these inconveniences, proposals have conventionally been made to prevent an increase in the temperature of a non-paper feeding area by cooling a part of a fuser roller pair corresponding to a non-paper feeding area. JP-A-05-181382 discloses an image forming apparatus provided with a cooling fan that, when more than a predetermined number of transfer materials having a set width smaller than the maximum width transportable by a rotating member (a fuser roller pair) in the axis direction thereof are transported within a predetermined time, blows cooling air onto a non-paper feeding area where the transfer material having the set width does not pass through, and a guide device (a duct) that guides the cooling air.
However, according to the method disclosed in JP-A-05-181382, blowing of air from the cooling fan is controlled by opening and closing the window provided near the non-paper feeding area of the fuser roller pair. This undesirably complicates the structure around the fuser. Besides, there is a need to make room for a duct around the fuser roller pair, making it impossible to achieve a space-saving image forming apparatus.
Moreover, the following problem arises when a large-sized sheet of paper is transported. The large-sized paper is easily wrinkled during transportation, often resulting in distortion of an unfused toner image formed on the paper. Furthermore, as shown in FIG. 10, in the case of a duplex-printing image forming apparatus that re-transports, instead of ejecting, a sheet of paper having an image on one side thereof to the image forming means to repeat image formation on the other side thereof, the sheet of paper that has an image on one side thereof and is re-transported inside the apparatus tends to be cooled more slowly than one that is directly ejected from the apparatus. This also slows cooling and hardening of the toner, causing problems such as adhesion between sheets of paper, print dropouts, or smudges at the time of duplex printing.
To overcome these problems, JP-A-09-146309 discloses a duplex-printing image forming apparatus provided with an intermediate tray that reverses a sheet of paper having an image on one side thereof, wherein a sheet of paper to which a toner image has been transferred is transported to a fuser while being stuck to the transportation means by air suction of the fan, and the sheet of paper placed on the intermediate tray is cooled by the air blown thereonto from the fan.
According to the method disclosed in JP-A-09-146309, transportation of a large-sized sheet of paper to which a toner image has been transferred can be stabilized by air suction of the fan, and the fan used for stabilizing the transportation of the paper can be shared as the fan used for cooling the paper at the time of duplex printing. This helps reduce adhesion between the sheets of papers, print dropouts, or smudges at the time of duplex printing. This also makes it possible to reduce the number of fans, and thereby achieve a reduction in noise as well as in cost. However, since the image forming apparatus disclosed in this Patent Document does not have the function of cooling a non-paper feeding area of a fuser roller pair, it is impossible to prolong the life of the fuser roller pair.